Air traffic control transponders are used in airplanes to respond to interrogations from air traffic control ground radar stations. The air traffic control radar transponder located in the airplane senses interrogation signals being transmitted from the ground and if the signal is a proper signal returns a response signal back to the ground. In order to minimize the amount of clutter on the radar oscilloscope that the ground operator watches, a system has been instituted known as side-lobe suppression for the air traffic control radar system. Basically, what this means is that the transponder on the aircraft only replies to the main lobe of the radar antenna pointing towards the airplane and not to any other extraneous radiations from the ground. Therefore, special circuitry has been incorporated in transponders which are referred to as side-lobe suppression circuits.
In order to accomplish side-lobe suppression, the ground radar transmits a special side-lobe suppression pulse along with other pulses to the aircraft during usual interrogation. The side-lobe suppression pulse is generally referred to as pulse P2 in the pulse train. The pulse train normally includes pulses P1, P2, and P3. Pulses P1 and P3 are transmitted by a directional antenna and the side-lobe suppression pulse P2 is transmitted by an omni-directional antenna located on the ground.
As previously mentioned, transponders now being used on aircraft have circuits provided therein recognizing the pulse P2 that is sandwiched between the pulses P1 and P3. When the pulse P2 is a certain amplitude compared to pulse P1, such causes circuitry associated with the transponder to be triggered indicating a true interrogation. However, if the side-lobe suppression pulse P2 is equal to or greater than pulse P1, then the transponder will not reply. When P1 is at least 9 db. greater than P2, the transponder must reply. So there is a region of 9 db. in between what is called a "threshold" decision region. In order to recognize the differences in amplitude between pulses P1 and P2, the transponders must be equipped with circuitry to make such distinction. There have been quite a few attempts at making this comparison, however, they normally incorporate complicated and expensive circuits. Attempts have been made to construct less expensive circuits, however, normally these circuits have difficulty differentiating over a very large dynamic range or a very large signal range accurately.